There is a compelling interest in determing the three-dimensional structures of several proteins that are involved in cardiovascular function and development. These proteins include two polypeptide growth factors, fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF), their cognate cell surface receptors, FGFR and PDGFR, and a cardiac muscle structural protein, troponin T (TNT). Milligram amounts of these target proteins will be purified from expression systems or natural sources in preparation for crystallization trials. Suitable single crystals will then be analyzed by conventional x-ray crystallographic techniques. In the case of the large transmembrane receptor molecules, functional domains of either the extracellular binding region or the intracellular tyrosine kinase region will be separately expressed and analyzed. The detailed binding interaction of growth factors with their respective receptors will be probed by cocrystallization of hormones/receptor-binding-domains. these complexes, when compared with the native forms of hormones and receptors, will reveal the conformational changes that are believed to trigger the mitogenic effect of the receptor catalytic domain, the intracellular tyrosine kinase. Various forms of the target proteins are being considered for crystallographic analysis. These include cellular isoforms (such as the homologous A and B chains of PDGF or the acidic and basic forms of FGF), homologs from other species (as the available Xenopus and human FGF molecules), distantly related proteins and oncogenic homologs (as the transforming homolog of PDGF-B p28sis of the simian sarcoma retrovirus) and variants of the target group that have been genetically manipulated. This latter group of proteins, site-directed mutants, depend on accurate tertiary structures of the native proteins to design new functions or structural attributes.